An active matrix substrate used in a liquid crystal display device or the like is provided with a switching element such as a thin film transistor (Thin Film Transistor: hereinafter referred to as “TFT”) for each pixel. As such a switching element, conventionally, a TFT in which an amorphous silicon film is used as an active layer (hereinafter referred to as “an amorphous silicon TFT”), and a TFT in which a polycrystalline silicon film is used as an active layer (hereinafter referred to as “a polycrystalline silicon TFT”) are widely utilized.
In recent years, as a material for the active layer of a TFT, it is proposed to use an oxide semiconductor instead of amorphous silicon or polycrystalline silicon (see Patent Documents No. 1 and No. 2, non-Patent Documents No. 1 to No. 3, and the like). Such a TFT is referred to as “an oxide semiconductor TFT”. The oxide semiconductor has higher mobility than amorphous silicon. For this reason, the oxide semiconductor TFT can operate at higher speeds than the amorphous silicon TFT. In addition, an oxide semiconductor film is formed by a process which is simpler than that for a polycrystalline silicon film, so that the oxide semiconductor film can be applied to a device which is required to have a large area.